This invention relates generally to digital VSB (vestigial side band) television receivers and specifically to a simple low cost system for coupling a digital VSB signal from any of a plurality of sources of a digital signal (such as an MPEGxe2x80x94Motion Picture Experts Groupxe2x80x94digital signal) to a digital VSB television receiver. While the description is directed to a VSB digital format, it should be understood that the invention in its broader aspects is not limited to a particular digital format.
The recently adopted Digital Television Standard specifies a VSB subsystem having a terrestrial broadcast mode (8 VSB) and a high data rate mode (16 VSB). There are other VSB modes available, namely 2 VSB, 4 VSB and 8 VSB (non terrestrial). The 8 VSB terrestrial mode has the data carrying capability of 4 VSB. (The Trellis coding that is added for the terrestrial environment creates the additional modulation levels.) The digital data signal for video is MPEG encoded and for audio is Dolby AC-3 processed and must be subjected to decompression before application to conventional video and audio circuitry. For terrestrial broadcasting, the data signal is: randomized; subjected to Reed-Solomon (R/S) type encoding for error correction; interleaved; Trellis encoded; multiplexed with segment sync and field sync; supplied with a DC pilot; subjected to pre-equalization filtering; modulated; and RF upconverted for transmission. The digital television receiver includes a tuner and a VSB demodulator for developing the baseband signal, which is in compressed form. The demodulated signal is applied to an equalizer for equalizing the received signal. The baseband signal is applied to a transport demultiplexer which directs the data to an appropriate MPEG decoder and a Dolby decoder for recovering the video and audio in proper form for application to video and audio processing circuitry. Digital signals may be supplied to digital receivers by over-the-air transmission sources. Digital signals from other sources, such as DVD (digital video disk) players, VCR""s (video cassette recorders), PC""s (personal computers), digital cable boxes, satellite receivers and the like will also be supplied to digital television receivers, as is the case for present analog television receivers.
In an analog system, it is well known that coupling the signal from a VCR or other video source to a baseband input of a television receiver (if one is available) yields results superior to those achieved by modulating the signal to RF channel 3 or RF channel 4 and supplying it to the tuner input of the television receiver. This baseband coupling, however, does not necessarily produce superior results in a digital environment in which case the distance between the baseband signal source and the television receiver must be kept relatively short. In an analog system, it is also well known to supply double sideband signals to a receiver.
In accordance with the invention described in U.S. application Ser. No. 09/300,299 filed Apr. 27, 1999, U.S. Pat. No. 6,559,898, a compressed baseband digital signal is encoded in a given format for digital transmission and is modulated onto an RF carrier for direct application via a cable or wireless link to the RF input of a digital television receiver. The RF signal, which is at a low power level and has a less-than-nominal bandpass, takes advantage of the front end signal processing that is built into the digital television receiver. Signal impairments, due to the less-than-nominal bandpass characteristic and noise introduced during transmission, that are below a given threshold are corrected by the correction circuitry in the digital television receiver front end. The low signal power, the corrective circuitry for the digital signal, and the benign environment of the communications link enable components and circuits of much lower tolerance (and cost) to be used in the encoding and modulating process implemented by the other sources described above. The result is an modulator that is very low cost and very effective in coupling a digital signal to a digital television receiver over an RF channel.
The present invention is directed to another arrangement for providing a digital signal to a digital receiver.
In accordance with a first aspect of the invention, a system for supplying a compressed digital signal to a digital television receiver comprises a source of compressed baseband signal, an encoder, first and second frequency shifters, and first, second, and third interpolators. The encoder encodes the baseband signal in a digital format, wherein the baseband signal has a frequency. The first frequency shifter shifts the frequency of the encoded baseband signal. The first interpolator interpolates an output of the first frequency shifter, the second interpolator interpolates an output of the first interpolator, and the third interpolator interpolates an output of the second interpolator. The second frequency shifter shifts a frequency of an output of the third interpolator.
In accordance with another aspect of the invention, a method of supplying a compressed digital signal to a digital television receiver comprises the following: encoding a baseband signal in a digital format, wherein the baseband signal has a symbol frequency; increasing the symbol frequency of the encoded baseband signal to produce an intermediate frequency signal, wherein the increasing of the symbol frequency of the encoded baseband signal includes three interpolations of the encoded baseband signal; and, modulating the intermediate frequency signal with a channel selection frequency signal so as to produce an output signal having the frequency of a selected channel.
In accordance with still another aspect of the invention, an electrical signal is produced by (i) encoding a signal in a digital television format, wherein the signal, has a symbol frequency f1, (ii) shifting the frequency of the encoded signal by f1/4, (iii) increasing the frequency of the shifted encoded signal to produce an intermediate signal, and (iv) modulating the intermediate frequency signal with a channel selection frequency signal so as to produce the electrical signal at the frequency of a selected channel.